1. Field of the Invention:
The present invention relates to novel crosslinkable starch ether derivatives.
2. Description of the Prior Art:
It is well known in the art that starch derivatives containing pendant ethylenically unsaturated groups may be crosslinked to obtain coatings for various materials such as paper, textiles and polymers. Typical of such ethylenically unsaturated starch derivatives are acrylate esters, allyl ethers (see J. P. Radley, "Starch and Its Derivatives", 4th ed., 1968 and Ind. Eng. Chem., 37, 201 (1945)) and starch acrylamide ethers (see U.S. Pat. No. 4,060,506).
Because these starch derivatives do not appreciably crosslink when cooked in water at any pH, they must be induced to crosslink using infrared or ultraviolet radiation, chemical agents such as cobalt naphthenate, or free radical initiators such as, e.g., peroxides, persulfates, azo initiators, etc. The crosslinking reaction employing free radical initiators may take place using any conventional thermally- or irradiation-induced free radical polymerization technique.
Induced crosslinking of starch derivatives as described above, however, has many disadvantages. For example, when crosslinking is induced by free radical initiators, the reaction is very difficult to control. In addition, many of the applicable free radical initiators will actually attack the starch, resulting in starch degradation. Allyl starch ethers, which can be crosslinked with the application of ultraviolet and infrared radiation as well as by free radical initiation, yield, upon crosslinking, coatings which have inferior properties such as non-homogeneity, inflexibility and poor water resistance. Consequently, there is a need in the art for a starch derivative which will crosslink upon damand without requiring inducement by irradiation or a chemical agent such as a free radical initiator.
N-(Alkoxymethyl)acrylamides are reactive crosslinking monomers which have been used as reagents for a number of applications. It is known that crosslinking through the N-(alkoxymethyl)amide group with amine, amide, hydroxyl or carboxyl groups is achieved under acidic conditions and/or elevated temperatures. In addition, it is known that N-(iso-butoxymethyl)acrylamide will condense directly with cellulose to split out alcohol to form a cellulose ether derivative having ethylenically unsaturated groups (see U.S. Pat. No. 2,837,512). An analogous starch derivative, of which the following formula is representative (but not definitive): ##STR1## wherein R and R' are monoorgano or hydrogen groups, is typically prepared by reacting a starch base with the appropriate N-(hydroxymethyl)acrylamide reagent at a pH less than 5 in the presence of an acid catalyst and polymerization inhibitor (see U.S. Pat. No. 4,060,506). As mentioned above, however, these derivatives require inducement in order to be crosslinked for use in coatings, etc. Although reaction of starch with acrylamide or N-alkyl-substituted acrylamide derivatives has been disclosed (see U.S. Pat. No. 3,135,738), reaction of a starch base specifically through the ethylenic linkage of N-(alkoxymethyl)acrylamides has not been investigated.
Accordingly, it is an object of the present invention to provide a novel crosslinkable starch ether derivative having no pendant ethylenically unsaturated groups.
It is a further object to provide a process for preparing a starch ether derivative which is crosslinkable at low pH in the absence of irradiation or a free radical initiator.